1. FIELD OF THE INVENTION
This invention relates to an exhaust manifold gasket inserted between the opposed joint surfaces of a cylinder head and an exhaust manifold and tightened and fixed with tightening bolts.
2. DESCRIPTION OF THE PRIOR ART
An exhaust manifold gasket for an engine which is inserted between a cylinder head and an exhaust manifold and tightened with tightening bolts has been provided so as to prevent an exhaust gas from leaking from the joint surfaces, i.e. contact surfaces of a cylinder head and an exhaust manifold. Various types of gaskets are being used; a typical gasket being made from asbestos.
In a high performance engine for automobiles which is being developed with a view to reducing the weight and dimensions thereof or increasing its output level, new techniques have recently been employed, and they cause problems to arise in some cases. The problems include an increase in the thermal expansion or thermal contraction, which occurs when the temperature varies of a cylinder head because of the employment of an aluminum alloy out of which the cylinder is formed so as to reduce the weight thereof, an increase in the thermal deformation of a gasket mounting surface, i.e. a mounting flange of an exhaust manifold because of the reduction of the wall thickness thereof, the breakage of a gasket because of the stress, a thermal load imparted to the gasket under certain conditions of thermal cycle during the use of the engine, and the leakage of a gas due to a sudden decrease of a sealing surface pressure. Under these circumstances, a method using a metal gasket as an effective sealing means instead of a conventional asbestos gasket has become generally known, and various types of metal gaskets have been developed.
For example, Japanese Utility Model Laid-Open No. 116149/1987 discloses a laminated type metal manifold gasket for engines. In this manifold gasket, laps are formed on the edges of gas passing holes, and seal beads are provided on the portions of two adjacent intermediate plates out of laminated metal plates which are around the exhaust holes therein, which intermediate plates are arranged with the beads thereon opposed to each other or facing in the opposite directions. A stainless steel plate is provided on the exhaust manifold-contacting surface of one outer plate, and the cylinder head-contacting surface of the other outer plate is subjected to a rust proof treatment (plating, chemical treatment or formation of a heat resisting film). All of these metal plates are combined unitarily into a laminated structure by a binding means, such as spot welding. In this manifold gasket, the thickness of the cylinder head-side outer plate is set larger than those of the remaining plates in the laminated structure, or the cylinder head-side outer plate is formed as a connecting plate joining together the gaskets corresponding to the exhaust ports of the engine.
Japanese Utility Model Laid-Open No. 61169/1990 discloses a laminated type metal manifold gasket. This gasket is inserted between the end surface of a mounting flange of an exhaust manifold and a side surface of a cylinder head in which exhaust gas discharge ports are opened, and consists of at least three metal plates laminated in a non-combined state without being fastened to one another. The metal plates positioned on both sides consist of beaded seal plates, and an intermediate metal plate a thin plate at least one surface of which is coated with a smooth surfaced layer of a heat insulating material, such as a ceramic material. In this gasket, the intermediate plate consists of a thin plate at least one surface of which is coated with a smooth-surfaced layer of a heat insulating material composed of a mixture of a ceramic material and a metal material, and the beaded plates positioned on both sides are provided with beads on the opposed sides thereof or on the non-opposed sides thereof.
Japanese Utility Model Laid-Open No. 61170/1990 discloses a laminated type manifold gasket. This gasket is inserted between the end surface of a mounting flange of an exhaust manifold and a side surface of a cylinder head in which exhaust gas discharge ports are opened, and consists of at least three thin plates laminated in a non-combined state without being fastened to one another. The thin plates positioned on both sides consist of beaded seal plates, and at least one surface of the intermediate plate a smooth-surfaced layer of a heat insulating material, such as a ceramic material. In this gasket, the intermediate plate consists of a heat insulating material composed of a mixture of a ceramic material and a metal material, and the beaded plates positioned on both sides are provided with beads on the opposed sides thereof. These beaded plates positioned on both sides may be provided with beads on the non-opposed side thereof as well instead of the opposed sides thereof.
A laminated type metal gasket in which a plurality of thin metal plates mentioned above are laminated has a high flexibility and a high thermal resistance, and comes to have a suitable compressibility by being provided with beads. The sealability of such a gasket rarely deteriorater with the lapse of time. Accordingly this gasket is used very advantageously for sealing a manifold which expands greatly at a high temperature.
Although the conventional metal gaskets have these advantages, they have the following problems. A conventional metal manifold gasket consists of metal plates, and it therefore has an extremely high heat conductivity. Accordingly, high heat is radiated from the engine to the exhaust manifold and the temperature gradient of the engine becomes high. Since the engine as a whole, exhaust manifold, cylinder head and cylinder head gasket are cooled rapidly, a thermal contraction load becomes large, so that, for example, cracks in the collecting portion of the exhaust manifold, cracks in the water hole portions of the cylinder head and cracks in the grommets of the cylinder head gasket occur. This causes various problems to arise which include a decrease in the sealing performance of the manifold gasket and a decrease in the endurance thereof.
In order that an engine has a high performance, a manifold having a high temperature gradient and excellent heat radiation performance is preferably provided. However, when the manifold is formed so as to have a higher temperature gradient, the cracks mentioned above occur. Let Ts equal the temperature gradient of the exhaust manifold, Tg the temperature of a portion in the vicinity of the portion into which the gasket is inserted, and Tc the temperature of the exhaust gas collecting portion. The equation of: EQU Ts=Tc-Tg
is then established. The temperature gradient Ts of the exhaust manifold is Ts=250.degree. C., for example, when a conventional asbestos gasket is used, Ts=325.degree. C. when a three-layer metal gasket is used, and Ts=375.degree. C. when a graphite gasket is used.
A maximum temperature of the exhaust manifold and engine mounting portion reaches about 700.degree.-800.degree. C. Therefore, when the gasket is formed out of a material other than a metal, it is very difficult to maintain the various characteristics thereof, such as compression resiliency, sealability, deformation resistance, fatigue resistance and strength while retaining the thermal resistance and heat insulating characteristics thereof. A metal gasket therefore has problems of eliminating these inconveniences and developing a method of forming a satisfactory metal manifold gasket.
Even the gaskets provided so as to solve these problems and disclosed in the above Japanese Utility Model Laid-Open Nos. 61169/1990 and 61170/1990 still have problems concerning the compression resiliency and heat shielding capability when the temperature increases to a high level due to an increase in the temperature of the exhaust gas, which is ascribed to an increased output of a recently-developed engine, and due to an increase in the flexure of the mounting surface of the exhaust manifold.